1. Field of the Invention
The present invention relates generally to a method and apparatus for a communication system. More particularly, the present invention relates to a method and apparatus for compensating for a timing synchronization error in a communication system.
2. Description of the Related Art
Generally, in a communication system, a Base Station (BS) of an indoor picocell has difficulty in accurately setting timing synchronization, compared with a BS of a macrocell. The reason is that while the macro BS sets timing synchronization using a Global Positioning System (GPS), the indoor picocell BS is unlikely to be able to use the GPS and thus has difficulty in setting timing synchronization using the GPS. For example, in a Mobile World Interoperability for Microwave Access (WiMAX) communication system, while timing synchronization accuracy of the macro BS is ±1, timing synchronization accuracy of the indoor picocell BS is ±3.5 μs˜±2.5 μs, which is about three times higher than the timing synchronization accuracy of the macro BS in error level. If a timing synchronization error between BSs is represented by θ, a timing difference θA−θB between received signals from two neighbor BSs, i.e., a BS A and a BS B, can be represented by θdiff. For example, if a range of θ is ±3.5 μs, a range of θdiff becomes ±7.0 μs.
An increase in the timing difference between received BS signals may give rise to various problems. One of the typical problems is that cell scanning performance of a User Equipment (UE) degrades, and another problem is that during Multicast and Broadcast Service (MBS) provision, an error may occur in timing synchronization setting and channel estimation by a UE.
First, reference will be made to FIGS. 1 and 2 to describe the problem that cell scanning performance of a UE degrades due to an increase in timing difference between received BS signals that the UE receives from a neighbor BS and a serving BS.
FIG. 1 shows a timing difference between received BS signals in a communication system according to the related art. In FIG. 1, an example of the communication system is a Mobile WiMAX communication system. FIG. 2 is a diagram showing a Carrier to Interference and Noise Ratio (CINR) estimation performance error caused by Inter Symbol Interference (ISI) in a communication system according to the related art.
Prior to a description of FIG. 1, it is to be noted that a UE determines whether to perform handover by performing a cell scanning operation of estimating a CINR using a preamble transmitted by a neighbor BS and comparing the estimated CINR with a CINR of a preamble transmitted by a serving BS. In the cell scanning operation, the UE sets timing synchronization with the serving BS, and then performs a correlation operation by Fast Fourier Transform (FFT). In this case, if a signal from a BS scanned by the UE is received ahead of a timing point which is set in the UE, ISI may occur as shown in FIG. 1. That is, in FIG. 1, since a received neighbor BS signal is received ahead of the timing point set in the UE, a signal of the next symbols received beginning from the point, at which the preamble signal is terminated, may cause ISI.
As shown in FIG. 2, the ISI causes a CINR estimation error in a CINR region where the CINR exceeds a predetermined threshold CINR. For example, it is noted in FIG. 2 that if a timing difference θdiff between received signals from a neighbor BS and a serving BS is 5.0 μs, a CINR floor occurs at 17.9 dB, and if a timing difference θdiff between received signals from a neighbor BS and a serving BS is 7.0 μs, a CINR floor occurs at 16.3 dB. In particular, when the Mobile WiMAX communication system uses a frequency reuse factor of 3, accurate CINR estimation performance is needed even in a relatively high CINR region of 10 dB or more as handover occurs even in a CINR region where the CINR exceeds a threshold CINR. In this case, the degradation of cell scanning performance may act as a fatal cause of reducing performance of the Mobile WiMAX communication system.
Second, reference will be made to FIG. 3 to describe the problem that during MBS provision, error may occur in timing synchronization setting and channel estimation by a UE. FIG. 3 is a diagram schematically showing the impact of a timing difference between received signals when a UE receives MBS in a communication system according to the related art.
Prior to a description of FIG. 3, it is to be noted that during MBS provision, a UE acquires fine synchronization by re-generating a preamble in a time domain and performing cross-correlation, and sets an FFT window on the basis of a peak path of a serving BS. In this case, if a Channel Impulse Response (CIR) of a serving BS falls behind a CIR of a neighbor BS, i.e., if a timing difference θdiff between received signals from a neighbor BS and a serving BS is less than 0 (θdiff<0) as shown in FIG. 3, then ISI may occur as described in FIG. 1. Due to the occurrence of ISI, a needed CINR increases, making it impossible to support a threshold Modulation and Coding Scheme (MCS) level, for example, an MCS level of 64-ary Quadrature Amplitude Modulation (QAM) ½ or more.
Accordingly, there is a need for a plan to compensate for a timing synchronization error to reduce a timing difference between received BS signals.